After only a somewhat recent introduction, optical fiber has had a meteoric rise as the predominant means of transmission media in voice and data communications. Optical fiber is manufactured by drawing glass fiber from an optical glass preform which is made by any of several well known processes. Afterwards, or as part of a tandem process, the drawn fiber is coated, cured, measured and taken up, desirably in an automatic takeup apparatus, on a spool to provide a package.
Typically, an optical glass fiber has a diameter on the order of 125 microns, for example, and is covered with a coating material which increases the outer diameter of the coated fiber to about 250 microns, for example. For single mode use, the glass fiber includes a core having a diameter of about 6.2 microns and a cladding system having a diameter of about 125 microns. The cladding system comprises inner and outer claddings. At least the outer portion of the cladding system is the precursor tube in which have been deposited materials to provide the core and the inner cladding when the tube is collapsed to form a preform.
An optical fiber package is used in operations such as ribboning, cabling, and rewinding and is used to ship optical fiber to other companies which further process the fiber. The optical fiber typically is used in voice and data communications systems, both commercial and military. For example, the package may be used in weapons systems in which it is used for guidance and for data communications. Such uses include communication lines between aircraft, between an aircraft and a ship, and between a projectile, such as a missile, and a control station at a launch site, for example. Optical fiber provides the advantages of increased data bandwidth, reduced weight and greater range than wire-guided systems of the prior art.
A typical optical fiber application in a weapons systems involves the packaging of a continuous length of optical fiber on a bobbin which is positioned inside a vehicle. Such a vehicle commonly is referred to as a tethered vehicle. In that application, optical fiber is payed out from a bobbin in the tethered vehicle. One end of the fiber is attached to operational devices in the vehicle, whereas the other end of the fiber is connected to a control or communications station at a launch site. During and after launch, two-way communication with the vehicle is conducted.
There are, however, certain disadvantages, not present in other forms of communication, in using optical fiber. Optical fiber is less robust than metallic conductors, rendering it subject to breakage. Aside from breakage, optical fiber communication performance may be degraded by microbends, which are totally determined by mode field radius, and macrobends in the fiber which are generated by bending or by other stresses to which the fiber may be subjected. Such damage to an optical fiber not only reduces the long-term durability of the fiber, but also causes losses in the strength and in the content of the optical signal. Likewise, physical or optical integrity may be affected adversely by any sharp bends which are experienced as the fiber pays out at immensely high speeds from its packaged configuration. In this usage, the fiber undergoes severe bending. In fact the bending radius may be on the order of a few millimeters. What is needed is an optical fiber that is resistant to small bends. Prior art single mode fiber normally has not been sufficiently bend resistant for deployment in tethered vehicles.
In order to overcome this problem, some have tried the use of dispersion shifted optical fiber, and more particularly, a dispersion flattened fiber. However, such design optical fibers are too bend sensitive and allow optical power to come out into the cladding.
The most predictable loss in a tethered vehicle link is the loss caused by the small radius bend during payout. Accordingly, what is sought after is a fiber in which the small radius bending loss is minimized while initial loss is held to a minimum. Also chromatic dispersion should not be a limiting factor for the system which is sought with suitable laser diode transmitters.
Other than optical requirements, high strength and tight dimensional control are important. Also, inasmuch as long lengths are required, it becomes important that the sought after fiber should be easily manufactured from a large preform.
What is sought after and what seemingly is not available in the prior art is an optical fiber which is suitable for use in tethered vehicle applications. It should be one which is easily manufacturable and which provides acceptable optical and mechanical performance notwithstanding its subjection to severe bending during payout.